This invention relates generally to rotary down the hole drills, and more particularly to towers (also called derricks) and cable feed systems thereon.
In a rotary head drilling system, a rotary powerhead moves up an down the length of a derrick, which supports this mechanism and the associated drill rods or drill pipe. The rotary powerhead applies torque to the drill rods and rotates the drill rods which in turn rotate the drill bit. A hydraulic cylinder or cylinders are mounted to the derrick structure and are retracted or extended moving sheaves or sprockets that are connected to the rotary powerhead by wire rope cables or roller chain.
In a conventional feed system, usually a single cylinder rod is employed, and the cylinder rod is in compression during the pulldown or pullback modes, rather than tension. This leads to rod buckling from compression. A single cylinder rod system may require long strokes, making galling and buckling of the rod a problem. Furthermore, with single rod systems, larger rod bore sizes are required to avoid buckling, resulting in heavy and expensive equipment.
The foregoing illustrates limitations known to exist in present rotary drill feed systems. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.
In one aspect of the present invention, this is accomplished by providing a rotary drill rig comprising: a drill platform; a tubular derrick for carrying drill machinery, the derrick supported on the drill platform in a vertical operating position; a rotary powerhead movable back and forth along the derrick for moving and rotating a drill string; a fixed sheave assembly on the derrick for supporting flexible cable thereon; a movable sheave assembly on the derrick for supporting the cable thereon and for moving back and forth along the derrick to move the power head back and forth along the derrick thereby moving a drill string up and down; a double rod, hydraulic cylinder assembly on the derrick for moving the movable sheave assembly back and forth, the cylinder assembly having a first and second piston rod reciprocable within a cylinder casing, the cylinder casing immovably fixed to the derrick, whereby the first and second piston rods are maintained in tension during operation thereof; the cable strung between the movable sheave assembly, the fixed sheave assembly and the powerhead, to provide a movement ratio measured between the power head and the first and second piston rods of at least 3 to 1; and means for supporting the movable sheave assembly on the derrick during movement thereof.
The foregoing and other aspects will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing figures.